Xenorhabdus spp., entomopathogenic bacteria symbiotically associated with the nematodes Neoaplectana and Heterorhabditis, occur in two forms. In general, only one form, designated the primary form, is transmitted into new hosts by the infective stage of the nematode. The significance of the relationship between the two forms has been examined with X. nematophilus, the symbiont of N. feltiae. The forms of X. nematophilus can be differentiated by their colony characteristics but by only two biochemical tests. The two forms of X. nematophilus are equally pathogenic when injected into the haemocoel of Galleria larvae. However, the primary form when injected into Galleria larvae with axenic nematodes provides better conditions for reproduction of the nematodes than the secondary form, for which a role has not been determined. Although the primary form readily converts to the secondary form in vitro and occasionally in vivo, the secondary form is usually stable. Possible causes of the instability have been investigated.

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Primary and secondary form variants of Xenorhabdus isolated from 21 strains (13 species) of Steinemematidae and Heterorhabditidae were tested for 240 biochemical and physiological characters. Primary form variants, isolated from the infective stage nematodes, could always be distinguished from the secondary by adsorption of neutral red from MacConkey agar. Lecithinase, antibiotic activity and/or adsorption of bromothymol blue were useful for distinguishing the variants of most strains. The variants of all strains also differed for other characteristics but the distinguishing characteristics varied from strain to strain. The importance of including both variants of each strain and of using appropriate methods in the study of Xenorhabdus taxonomy was demonstrated.

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